1. Field of the Invention
The present invention relates to lasers, and more particularly, to a new means and method for exciting an electric discharge in gas lasers, especially high-power gas lasers.
Electric glow discharges in superatmospheric high-power gas lasers (such as the rare-gas halide lasers) are basically unstable and can only be initiated by providing a uniform predischarge field and a uniform preionization level in the gas. Then when established, a distributed glow can be maintained for only short periods, generally a small fraction of a microsecond, particularly in gas mixtures containing electro-negative species such as F.sub.2, O.sub.2, NF.sub.3, SF.sub.6, and HCl, for example. The glow phase is terminated by the development of cathode streamers which later coalesce into one or more bright arcs. During the transition phase, streamers and glow may coexist, and effective laser excitation take place, provided the streamers do not extend over the entire cathode-to-anode gap and the discharge impedance has not collapsed to a very low value as a result. When the discharge impedance becomes very small due to arcs or streamers, the electrical energy oscillates rapidly between the circuit inductance and capacitance with practically no power dissipation in the laser gas.
Therefore, efficient excitation of the gain medium occurs only during a very short transition phase of the discharge, and this is believed to be the main reason for the low efficiencies (around 1%) observed with self-sustained rare-gas halide lasers using a conventional TE-laser geometry. Best efficiencies have been achieved with lasers having fast discharge networks with low impedance.
The usual TE-laser geometry results in a large cross-sectional discharge area and a short discharge gap length, thus giving a low-resistance positive column across the gap from cathode to anode. In scaling up to larger sizes, the impedance matching problem tends to worsen.
It is therefore an object of the present invention to provide a laser excitation scheme where an electric discharge in the laser gas is stable over a much longer time period than heretofore attainable. A further object is to provide such a gas laser excitation with only a single laser excitation power pulse, i.e., without separate preionization circuitry or the like. Ancillary objects are to provide an excitation system having much greater electric discharge efficiency than before and wherein the discharge impedance is many times greater than previously.
2. Description of the Prior Art
The usual TE-laser geometry features two relatively large flat electrodes in the laser gas volume spaced on opposite sides of the optical cavity center-line, transversely of the longitudinal direction of the beam. This is illustrated in FIG. 1 of U.S. Pat. No. 3,662,284, for example. Further, this same patent discloses using a corona-type discharge between an auxillary electrode and a main electrode to provide electrons and ultraviolet radiation for preionizing a high-pressure gas mixture in the main discharge gap of a transverse discharge laser prior to initiation of the main discharge.
The above-cited patent and an article titled "Doubled Discharge Excitation for Atmospheric Pressure Carbon Dioxide Lasers," The Review of Scientific Instruments, Vol. 41, pp. 1578-1581, Nov. 1970, employ a so-called "frustrated" discharge through a dielectric in conjunction with a separate predischarge circuit.
The principle of triggering a lonitudinal glow discharge by localized corona discharges through a dielectric wall is described in U.S. Pat. Nos. 4,010,397 and 4,075,537. These techniques apply specifically to promote easy discharge initiation in flashlamps containing xenon at low or only moderately high pressures.
While it is possible that more pertinent prior arts exist, Applicant's search is believed to have been conducted with a conscientious effort to locate and evaluate the most relevant art available at the time, but this statement is not to be construed as a representation that no more pertinent art exists.